A propellent charge in tablet or disc form is used as the combustible gas-producing material in the gas generators currently used for inflatable impact cushions (also called "airbags"). On combustion this charge generates the utilizable or compressed gas to inflate the impact cushion. Generally speaking, the disadvantage involved in the combustion of solid gas-producing materials lies in the very high proportion of slag which arises on combustion, and accounts for more than 50% of the propellent charge composition that is used. Because of the slag and dust that are formed on combustion, complex filter stages are required in the gas generator in order to retain slag and dust particles. Otherwise, if these particles escaped the impact cushion could be damaged and the occupants put at risk.
Gas generators with compressed gases or air are available as an alternative to generators containing combustible solids. Very high boost pressures are required to form a sufficient volume of gas, as cooling takes place when the gases escape and no gain in volume is achieved by exothermic reaction as in the case of solid mixtures.